Re: [PATCH 2/2] mm: shmem: improve the tmpfs large folio read performance

[Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx>]

On 2024/10/18 00:48, Yang Shi wrote:
> On Wed, Oct 16, 2024 at 8:25 PM Baolin Wang
> <baolin.wang@xxxxxxxxxxxxxxxxx> wrote:
> > On 2024/10/17 01:33, Yang Shi wrote:
> > > On Wed, Oct 16, 2024 at 8:38 AM Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote:
> > > > On Wed, Oct 16, 2024 at 06:09:30PM +0800, Baolin Wang wrote:
> > > > > @@ -3128,8 +3127,9 @@ static ssize_t shmem_file_read_iter(struct kiocb *iocb, struct iov_iter *to)
> > > > >                 if (folio) {
> > > > >                         folio_unlock(folio);
> > > > >
> > > > > -                     page = folio_file_page(folio, index);
> > > > > -                     if (PageHWPoison(page)) {
> > > > > +                     if (folio_test_hwpoison(folio) ||
> > > > > +                         (folio_test_large(folio) &&
> > > > > +                          folio_test_has_hwpoisoned(folio))) {
> > > >
> > > > Hm, so if we have hwpoison set on one page in a folio, we now can't read
> > > > bytes from any page in the folio?  That seems like we've made a bad
> > > > situation worse.
> > >
> > > Yeah, I agree. I think we can fallback to page copy if
> > > folio_test_has_hwpoisoned is true. The PG_hwpoison flag is per page.
> > >
> > > The folio_test_has_hwpoisoned is kept set if the folio split is failed
> > > in memory failure handler.
> >
> > Right. I can still keep the page size copy if
> > folio_test_has_hwpoisoned() is true. Some sample changes are as follow.
> >
> > Moreover, I noticed shmem splice_read() and write() also simply return
> > an error if the folio_test_has_hwpoisoned() is true, without any
> > fallback to page granularity. I wonder if it is worth adding page
> > granularity support as well?
>
> I think you should do the same.

OK. Let me have a detailed look.

> > diff --git a/mm/shmem.c b/mm/shmem.c
> > index 7e79b6a96da0..f30e24e529b9 100644
> > --- a/mm/shmem.c
> > +++ b/mm/shmem.c
> > @@ -3111,9 +3111,11 @@ static ssize_t shmem_file_read_iter(struct kiocb
> > *iocb, struct iov_iter *to)
> >
> >           for (;;) {
> >                   struct folio *folio = NULL;
> > +               struct page *page = NULL;
> >                   unsigned long nr, ret;
> >                   loff_t end_offset, i_size = i_size_read(inode);
> >                   size_t fsize;
> > +               bool fallback_page_copy = false;
> >
> >                   if (unlikely(iocb->ki_pos >= i_size))
> >                           break;
> > @@ -3127,13 +3129,16 @@ static ssize_t shmem_file_read_iter(struct kiocb
> > *iocb, struct iov_iter *to)
> >                   if (folio) {
> >                           folio_unlock(folio);
> >
> > -                       if (folio_test_hwpoison(folio) ||
> > -                           (folio_test_large(folio) &&
> > -                            folio_test_has_hwpoisoned(folio))) {
> > +                       page = folio_file_page(folio, index);
> > +                       if (PageHWPoison(page)) {
> >                                   folio_put(folio);
> >                                   error = -EIO;
> >                                   break;
> >                           }
> > +
> > +                       if (folio_test_large(folio) &&
> > +                           folio_test_has_hwpoisoned(folio))
> > +                               fallback_page_copy = true;
> >                   }
> >
> >                   /*
> > @@ -3147,7 +3152,7 @@ static ssize_t shmem_file_read_iter(struct kiocb
> > *iocb, struct iov_iter *to)
> >                           break;
> >                   }
> >                   end_offset = min_t(loff_t, i_size, iocb->ki_pos +
> > to->count);
> > -               if (folio)
> > +               if (folio && likely(!fallback_page_copy))
> >                           fsize = folio_size(folio);
> >                   else
> >                           fsize = PAGE_SIZE;
> > @@ -3160,8 +3165,13 @@ static ssize_t shmem_file_read_iter(struct kiocb
> > *iocb, struct iov_iter *to)
> >                            * virtual addresses, take care about potential
> > aliasing
> >                            * before reading the page on the kernel side.
> >                            */
> > -                       if (mapping_writably_mapped(mapping))
> > -                               flush_dcache_folio(folio);
> > +                       if (mapping_writably_mapped(mapping)) {
> > +                               if (unlikely(fallback_page_copy))
> > +                                       flush_dcache_page(page);
> > +                               else
> > +                                       flush_dcache_folio(folio);
> > +                       }
> > +
> >                           /*
> >                            * Mark the page accessed if we read the beginning.
> >                            */
> > @@ -3171,7 +3181,10 @@ static ssize_t shmem_file_read_iter(struct kiocb
> > *iocb, struct iov_iter *to)
> >                            * Ok, we have the page, and it's up-to-date, so
> >                            * now we can copy it to user space...
> >                            */
> > -                       ret = copy_folio_to_iter(folio, offset, nr, to);
> > +                       if (unlikely(fallback_page_copy))
> > +                               ret = copy_page_to_iter(page, offset,
> > nr, to);
> > +                       else
> > +                               ret = copy_folio_to_iter(folio, offset,
> > nr, to);
> >                           folio_put(folio);
> >                   } else if (user_backed_iter(to)) {
> >                           /*
>
> The change seems fine to me.

Thanks.